In the state of the art, different treatment methods having the goal of refraction correction of the human eye are known. In this connection, it is the goal of the operation methods to modify the cornea in a targeted manner in order to thereby influence the refraction of light. Multiple operation methods are used for this purpose. Currently the most widespread method is what is called laser in-situ keratomileusis, also abbreviated as LASIK. In this connection, first a cornea flap is detached from the cornea surface on one side, and folded over to the side. Detaching of this flap can take place by means of a mechanical microkeratome or also by means of what is called a laser keratome, such as that sold by Intralase Corp., Irvine, USA, for example. After the flap has been detached and folded to the side, use of an excimer laser that wears away the corneal tissue thereby exposed under the flap, by means of ablation, is provided during the LASIK operation. After the volume that lies below the cornea surface has been evaporated in this manner, the cornea flap is folded back into its original position.
Use of a laser keratome for freeing the flap is advantageous, as compared with a mechanical blade, because the risk of infection is reduced and, at the same time, the incision quality is improved. In particular, the flap can be produced with a much more constant thickness if laser radiation is used. Also, the incision is generally smoother, and this reduces the risk of subsequent optical disturbances caused by the boundary surface that remains even after the operation. However, it is a disadvantage of this method that two different treatment apparatuses have to be used, for one thing, specifically, the laser keratome for detaching the flap, and for another, the laser that evaporates the corneal tissue.
These disadvantages are eliminated in a method that was recently implemented by Carl Zeiss Meditec AG and is abbreviated with the designation FLEX. In this method, an incision geometry is formed in the cornea of the eye by means of a femtosecond laser, which separates a cornea volume (called a lenticule) in the cornea. This is then manually removed by the operator, after the flap that covers the lenticule has been folded to the side. The advantage of this method lies, for one thing, in that the incision quality is improved once again by means of the use of the femtosecond laser.
For another thing, the eximer laser is no longer used, and only one treatment apparatus is required.
An expansion of the FLEX method is the SMILE method, in which the separated lenticule is removed through a small opening incision, and thereby the flap incision of the FLEX method can be eliminated.
In the production of incision surfaces in the cornea by means of laser radiation, the optical radiation effect is usually utilized in that an optical perforation is produced. Also, it is known to introduce individual pulses, the energy of which lies below a threshold value for an optical perforation, into the tissue or material in such a covered manner that material or tissue separation is achieved in this way, as well. This concept of incision production in the corneal tissue allows a great variety of incisions.
In the laser surgery operation methods described, an incision remains in the cornea as the result of the treatment. The incision is no longer visible with the naked eye after a short time, but never heals, due to the particular nature of the cornea of the eye, because in this regard, the cornea of the eye is “dead” tissue. The tissue parts above an incision are no longer firmly connected with the tissue parts below the incision, because of the incision.
However, a need for after-treatment can occur, specifically if the result of the previous operation is not yet satisfactory with regard to the refraction correction, or if the previous operation could not be sufficiently concluded for some reason (for example due to discontinuation of the operation).
In the event of an insufficient refractory correction, it is known, for the excimer-laser-based LASIK operation, to lift the cornea flap up once again for the after-treatment, and to remove further corneal tissue by ablation.
However, this approach cannot be used for the FLEX method, because the incision in the cornea from the first treatment is not easily accessible or evident. The flap would have to be folded up again in order to remove the material that lies underneath, and the femtosecond laser is not intended for this; further work would have to be performed using an excimer laser. This approach also cannot be implemented in the SMILE method, because the incision from the first treatment, which runs in the interior of the cornea, is no longer accessible or evident.
The same holds true for an interrupted laser-based operation.
A first solution for this problem is described in DE 10 2007 019814. In this solution a planning device that has an interface for supplying cornea data that contain information about pre-operative steps generated in a previous ophthalmic surgery operation, and has calculation means for establishing a cornea incision surface that delimits the cornea volume to be removed, where the calculation means establish the cornea incision surface on the basis of the cornea data and generate a control data set for controlling the laser device for the cornea incision surface.
With the solution described there, however, it is not ensured that further treatment will take place at the desired location, because displacements or rotations of the eye can occur between the completion of the first treatment and its resumption or continuation. In order to avoid this problem, it is proposed, in DE 102007019814, to place the new incision(s) in such a manner that they are guaranteed not to intersect the original incisions, i.e., either completely posterior or completely anterior to the original incision.
This cannot always be implemented, because of the geometry and, in particular, the required stability of the cornea.
In DE 10 2008 056 488, it is therefore proposed to determine the original incision by means of a special detector. OCT (optical coherence tomography) or a confocal detector are disclosed as suitable detectors. Therefore this solution requires significant effort/expenditure for the additional detector.